A conventional method of implementing a magnetic field sensor using a MOSFET device is with the use of a special MOSFET having split drains in addition to the source and gate. A bias voltage applied to the device causes current flow between the source and the drains. A magnetic field perpendicular to the surface of the MOSFET causes a Lorentz deflection of the current flow in the gate channel so that current is deflected towards one or the other of the two split drains. The resultant imbalance in the drain current is used to sense the magnetic field strength. This current imbalance is typically measured by sensing the difference in voltage developed on two load resistors using a differential amplifier connected directly to the outputs of the split drains of the MOSFET. Improved sensitivity on the order of 10.sup.3 V/AT has been achieved by using the split drain approach, which is approximately a factor of 10 improvement over magnetic field sensitivity realized with use of conventional Hall Effect MOSFET devices.
A disadvantage of the use of load resistors in a conventional split drain MOSFET structure is that an undesirable noise component is introduced in the output signal. A load resistor has what is known as a "Johnson noise" current component associated with it, where the mean-square noise current is given by: EQU I.sup.2.sub.n =4kTB/R.sub.L ( 1)
in which k is Boltzman's constant, T is the absolute temperature, B is the noise bandwidth and R.sub.L is the load resistor value. The noise current of equation (1) exists independently of whether or not a signal current is present. Therefore, the use of a conventional split drain MOSFET structure as a magnetic field sensor inherently introduces an adverse noise component that limits the sensitivity of the sensor.
Undesirable signal noise is also introduced in the conventional MOSFET split drain sensor as a result of the receiver used with the sensor. A common method of sensing low level current signals is to use either a transimpedance receiver (amplifier) or a high impedance current sensing receiver. The mean-square noise current associated with both methods is given by the expression: EQU I.sup.2.sub.n =4kTB/R.sub.L +e.sup.2.sub.na B/R.sup.2.sub.L +[4/3].pi..sup.2 e.sup.2.sub.na C.sup.2 B.sup.3 ( 2)
with the symbols used being as noted above with respect to equation (1), and where e.sup.2 na is the amplifier equivalent mean-square input noise voltage, and C is the capacitance at the sensing node (with contributions from 1/f and shot noise being neglected).
Graphs wherein the mean-square noise current per unit frequency from equation (2) is plotted as a function of the noise equivalent bandwidth, B(MHz), demonstrate that the resistor noise dominates at low bandwidths and that the mean-square noise current per unit frequency due to the resistor is inversely proportional to the resistor value. Therefore, increasing the value of the load resistor reduces the mean-square noise current. However, in a transimpedance amplifier this increase can cause the amplifier to oscillate, while in the case of a high impedance receiver, a large equalization ratio is required. Therefore, both receiving methods suffer specific disadvantages.
It will be appreciated from the foregoing that even with the improved sensitivity provided by this type of split drain MOSFET structure, the use thereof as a magnetic field sensor is limited by noise inherent in both the load resistor and the sensing circuitry of the device. Thus, while prior art MOSFET magnetic field sensors of the type described above are satisfactory for many applications, such sensors do not, in general, provide the advantages of the invention described below.